1. Field of the Invention
The present invention relates to a garnet ferrite for non-reciprocal circuit devices used in high-frequency bands such as the microwave band, a non-reciprocal circuit device including the same, and a method for preparing the same. In particular, the present invention relates to a technique for reducing the insertion loss in high-frequency bands.
2. Description of the Related Art
Mnxe2x80x94Zn ferrites, Nixe2x80x94Zn ferrites, lithium ferrites, YIG ferrites, and the like have been used as magnetic materials for high frequencies.
Also, in radio communication apparatuses, such as cellular phones, non-reciprocal circuit devices using such a high-frequency magnetic material are provided between an antenna and an amplifier in order to stabilize the operation of the amplifier and to prevent cross modulation.
In those magnetic materials, a YIG ferrite in which part of the composition Y3Fe5O12 is replaced with Gd or Al has been put into practical use, and is well known as a material capable of exhibiting a low insertion loss and excellent saturation magnetization. Supposedly, the saturation magnetization 4 xcfx80Ms and the temperature coefficient (a) of the saturation magnetization 4 xcfx80Ms can be varied by controlling the substituted Gd or Al contents, and therefore, the YIG ferrite can exhibit various 4 xcfx80Ms values suitable to the frequency. In addition, when the ferrite is used in combination with a permanent magnet, it can advantageously compensate for the temperature characteristics of the magnet. Thus, the YIG ferrite is described as a material capable of being applied to stable non-reciprocal circuit devices, such as isolators and circulators.
The inventors have conducted research for use of the YIG ferrite as a non-reciprocal circuit device used in high-frequency bands of 500 MHz or more, and particularly on materials to reduce insertion loss, and have consequently achieved the present invention.
The YIG ferrite is generally prepared by calcining a raw mixture containing yttrium (Y) oxide powder, iron (Fe) oxide powder, and gadolinium (Gd) oxide power which have been weighed so as to form a desired composition, followed by pulverizing with a pulverizer, such as a ball mill or an attritor, and compacting the pulverized material into a desired shape. Finally, the material is fired and, thus, a non-reciprocal circuit device is completed.
Based on this method, the inventors have conducted research on the YIG ferrite. As a result, the inventors have found that it is difficult, using the YIG ferrite, to produce a non-reciprocal circuit device having the desired high-frequency characteristics, even if the YIG ferrite is prepared by weighing the powder materials so as to form a desired composition. According to this finding, the inventors reviewed the processes of this method, and found that, if the internal wall, pulverizing blade, or balls of the pulverizer contain elemental Fe, part of the Fe moves into the calcined material during pulverizing and mixing of the calcined material, and consequently, the resulting composition of the YIG ferrite cannot satisfy the stoichiometric requirements. This is likely to degrade the high-frequency characteristics.
Accordingly, an object of the present invention is to provide a garnet ferrite capable of realizing a low insertion loss in a high-frequency band of more than 500 MHz.
Another object of the present invention is to provide a garnet ferrite capable of realizing a low insertion loss in a high-frequency band of more than 500 MHz and which has a ferromagnetic resonance with a small half-width.
Still another object of the present invention is to provide a method for preparing a garnet ferrite having a desired composition with reliability.
According to an aspect of the present invention, a garnet ferrite used for a non-reciprocal circuit is provided. The garnet ferrite contains Fe in an amount 0.5% to 5% lower than the value derived from stoichiometry.
By reducing the Fe content to a value lower than the stoichiometric value, the resulting garnet ferrite will be non-stoichiometric and can exhibit a low insertion loss in a high-frequency band of more than 500 MHz.
Preferably, the Fe content is 1% to 3% lower than the value derived from stoichiometry.
By setting the Fe content 1% to 3% lower than the stoichiometric value, the resulting garnet ferrite can exhibit a low insertion loss in a high-frequency band of more than 500 MHz and an excellent ferromagnetic resonance half-width. An excessively reduced Fe content disadvantageously increases the ferromagnetic resonance half-width.
Preferably, the composition of the garnet ferrite is expressed by the formula A3B5O12, wherein A represents Y or Y and Gd, and B represents Fe or Fe and at least one element selected from the group consisting of Al, In, and Mn.
Thus, an excellent garnet ferrite containing YFeO, Gd substituted for part of the Y of the YFeO, or Al, In, or Mn substituted for part of the Fe of the YFeO can be provided. The garnet ferrite can exhibit a low insertion loss in a high-frequency band of more than 500 MHz and an excellent ferromagnetic resonance half-width.
The composition may be expressed by the formula YxGd3-xAl0.5Fe4.5O12, wherein X is 0 or more and less than 3.
Preferably, the composition is expressed by the formula Y3Fe5-5yAl5yO12.
This composition ensures a garnet ferrite capable of exhibiting a low insertion loss and a small ferromagnetic resonance half-width.
In addition, part of the Fe may is replaced with Mn in an amount equivalent to 0.1% by weight or less of MnO.
Mn is added to prevent oxidation, and it contributes to the reduction of the ferromagnetic resonance half width. However, an excessive Mn content increases the ferromagnetic resonance half-width.
According to another aspect of the present invention, a method for preparing a garnet ferrite used for a non-reciprocal circuit device, the garnet ferrite containing Fe in an amount 0.5% to 5% lower than the value derived from stoichiometry. The method comprises the steps of: mixing raw materials having a desired composition; calcining the raw materials to form a calcined material; pulverizing and mixing the calcined material; and subjecting the calcined material to forming and firing. The step of mixing the raw materials and the step of pulverizing and mixing the calcined material are performed using a pulverizer whose portion coming into contact with the raw materials or the calcined material does not contain Fe.
Thus, only the Fe content of the garnet ferrite is reduced to a value lower than the value derived from stoichiometry. The resulting garnet ferrite can exhibit a low insertion loss and a small ferromagnetic resonance half-width.
The pulverizer may be a ball mill or a planetary mill whose portion coming into contact with the raw materials does not contain Fe.
If the internal wall or balls of the ball mill are formed of an Fe alloy, part of the Fe is likely to move into the material mixture while the material mixture is in contact with the balls or the internal wall, in the pulverizing and misting step. The same goes for the planetary mill, and part of the Fe contained in the blade is likely to move into the material mixture while the blade is in contact with the material mixture. By using balls formed of alumina, zirconia, or the like and a metal core formed of calcium titanate or the like, the migration of Fe can be prevented.
According to another aspect of the present invention, a non-reciprocal circuit device is provided. The non-reciprocal circuit device includes a magnetic assembly, a magnet for applying a direct current magnetic field to the magnetic assembly, a matching capacitor, and a yoke for joining the magnetic assembly, the magnet, and the matching capacitor. The magnetic assembly includes a main body having a garnet ferrite containing Fe in an amount 0.5% to 5% lower than the value derived from stoichiometry and a plurality of center conductors disposed on the upper surface of the main body. The center conductors intersect each other at a predetermined angle with electrical insulation.
The non-reciprocal circuit device can exhibit a low insertion loss in a high-frequency band of more than 500 MHz.